239 Multilin 3

[jacphi]

i am using a multilin 239 relay in a MCC to control a motor 185 kw 480v motor. the ground ct used is having a ratio 50:0.025A. the system is solidly grounded. the instalation manual for multilin 239 says that this ct can be used only for resistance grounded system. for solidly grounded it should be 50:5A.

http://www.geindustrial.com/cwc/products?pnlid=6&famid=31&catid=218&id=239&typeId=3

i would like to know if i use this ct(50:0.025a) in a solidly grounded system with a short circuit current of 85 KA is there a problem???

 

[PWR]

If you have a solidly grounded system, you would be able to set a more realistic ground fault pickup value by using a residual connection of the phase CTs. With the 2000 to 1 CT I believe you will find the allowable menu maximum primary ground fault current value will still be low enough that you could have nuisance tripping on a solid grounded system. If you don't want to get another ground CT, I would definitely choose the residual connection that is described in the 239 manual.

 

[jbartos]

Suggestion to jacphi (Electrical) Oct 2, 2003 marked ///\\i am using a multilin 239 relay in a MCC to control a motor 185 kw 480v motor. the ground ct used is having a ratio 50:0.025A. the system is solidly grounded. the installation manual for multilin 239 says that this ct can be used only for resistance grounded system. for solidly grounded it should be 50:5A.

http://www.geindustrial.com/cwc/products...

i would like to know if i use this ct(50:0.025a) in a solidly grounded system with a short circuit current of 85 KA is there a problem???
///CT (50:0.025A) should not be used with the solidly grounded system since it is intended for the resistance grounded system. The resistance grounded system is substantially different from the solidly grounded system.\\

 

[jbartos]

Suggestion: The solidly grounded system should use the CT for residual connection. CT should be rated to withstand 6xFLA or higher, e.g. 12xFLA.
85kA faults will be tripped by the upstream motor protection. 85kA fault will momentarily saturate the CT for solidly grounded system with rating X:5A (in residual connection). The solidly grounded system CT, with some rating, I.E. X:5A, must be able to momentarily withstand 85kA. Check with the CT manufacturer. However, 85kA appears to be on a high side. Is 85kA rms or maximum peak?

 

[Kiribanda]

Hi jacphi,

I think – in agreeable with jbartos- this is going to be another wrong application.

Zero sequence (Donut c/t) ct connection (50 GS) is normally used for high resistance grounded systems. Since the earth fault current is limited in the range of 2-5 Amp, on those systems, by having a very low secondary current in the c/t (50/ 0.025) it is possible to increase the sensitivity of the relay operation. Then it will respond even for very small earth leakage currents. If the donut c/t is connected in the “CORRECT” way by passing through it “ONLY” the phase conductors there is no reason for it to trip for spurious earth fault currents in the vicinity.

But if we use the same c/t on a solidly grounded system where we have got very high earth fault currents, it is possible to get the c/t saturated and hence the objective is lost. If you make a rough calculation for the possible earth fault current magnitude at the end of the motor cable with a 480 V/85 kA fault level at the feeding point, you will definitely end up with at least 60 kA with a reasonable LV cable length and size. Then what will happen to the 50/0.025 c/t when there is a fault of 60 kA at the cable end (motor terminal box)? For your information even if you refer the IEEE Buff book it strictly recommends to not to use zero seq. earth fault protection schemes due to similar reasons.

May I divert the issue in some other direction! ( 480 V/ 185 kW motor )

1) Are you going to open the LV motor contactor by this earth fault trip signal for an earth fault current in 60 kA range during an earth fault which is possible in your solidly grounded system?

2) Can the LV contactor break such a high fault rating?

3) Does the relay has the facility to block the earth fault trip command for a while till the motor LV main fuses or LV circuit breaker handles that high current and open the circuit?

Note: It seems to me that you are going to apply a motor control relay suitable for a 4160 V high resistance system on a LV contactor system.

Regards!
Kiribanda

 

[jacphi]

from the reply of jbartos & Kiribanda it is clearly understood that we cannot use the 50:0.025A ct on a solidly grounded system. thanks for your immediate response.

for the questions asked by kiribanda
1. during earth fault, the auxiliary of the multilin is connected to the breaker. this will trip the breaker via the shunt trip and not the contactor.
2. the controlgear as a whole is rated for 100ka. so are the components used.
3. i suppose multilin is a very advanced relay with provision to block the earth fault trip command for a while.

regards

 

[buzzp]

In regards to question/answer 3, the unit should absolutely not have provisions for tripping quicker than the breaker on a short circuit. If it does, then I would have real concerns. I know it has provisions for 'delaying' this trip signal until a short circuit device handles the fault. However, as part of UL requirements, it can not have the ability to trip quicker on a short. This is generally assumed because of the CT's used (saturate at high levels of current faking the relay into thinking the current is lower than it actually is) and also the minimum trip time.

 

[Kiribanda]

Hi jacphi,

Thanks for the feed back.

I personally feel that a LV motor controlgear arrangement with

1)a LV power circuit breaker or MCCB with shunt trip provision

2)a Multilin 269 motor protective relay

3)Associated control supply provision, cubicle space etc

seems to be expensive for a LV 185 kW motor just to ensure earth fault protection.

Suggestion:
We should not forget that the magnitude of the earth fault current purely depends on the zero sequence impedance up to the fault location. So when we make provisions to reduce the zero sequence impedance in the field by proper installtion of the motor/ cable arrangement, I feel we can simply use MCCB/ HRC Fuse and Contactor arrangement for a LV 185 kW motor which is cheaper and simple.

Regards!
Kiribanda ;-)

 

[jbartos]

Suggestion: The 85kA might substantially be reduced by using current limiting fuses. If arcing happens at 85kA, it will cause a noticeable destruction to motor and its upstream hardware.